In-Ear MonitorOverreview

64 Audio U4s

The cheapest 64 Audio, and the one its own reviewers keep picking over the $2,000 flagship — with four screw-in modules that quietly settle most of the arguments about it.

13 sourcesUpdated
$1,199Typical US price

Score aggregates reviewer opinions across topics.

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64 Audio U4s

The short version

What to know.

A bass-led hybrid that reviewers rate very highly and describe very differently. The agreement is broad and unusually firm: the dynamic-driver low end is the headline and is praised almost without exception, macro-dynamics are a standout, the tuning is deliberately coloured rather than neutral, it runs off anything, and channel matching measures excellent. The disagreement is just as consistent, and most of it traces to one variable. The Apex module fitted sets the bass shelf across roughly a 6 dB range, sets isolation between about −10 and −20 dB, and trades stage width against imaging focus — so writers using different modules describe different in-ears. On top of that, the ear gain peaks early, near 2 kHz rather than 3 kHz, which pulls upper-mid vocals back: some hear that as relaxed and non-shouty, others as vocals losing their presence. Treble, soundstage, fit and finish all split along similar lines.

About this product

The universal (U) version of 64 Audio's A4s custom monitor, launched in March 2023 as the cheapest universal in the range — below the U6t, and well below the U12t. A hybrid: one dynamic driver plus two balanced armatures and a tia tweeter, not the all-BA arrangement of the U6t/U12t. Every cited review judges it at its $1,099 launch price; 64 Audio and most dealers now list it at $1,199.

When the U4s arrived in March 2023 it was the least 64 Audio had ever charged for a universal in-ear: $1,099 for a four-driver hybrid carrying all three of the company's house technologies — the tubeless tia tweeter mounted in the nozzle, the Apex pressure-relief modules, and LID, which flattens the impedance so the tuning holds steady off a phone or a desktop stack alike.

Reviewers reached for the comparisons immediately, and kept arriving at an awkward answer for 64 Audio: the cheap one is the one they wanted. It undercut the $1,700 Nio outright, and more than one writer admitted it made the $2,000 U12t harder to justify. What nobody quite agrees on is what the U4s sounds like — because four little aluminium plugs ship in the box, and which one is screwed in changes the answer.

A good fit for

  • Anyone who wants dynamic-driver bass — depth, weight and physical slam — without giving up treble extension
  • Listeners who like to tune by hardware: four modules cover roughly 6 dB of bass shelf and swap in thirty seconds, no EQ needed
  • People who find sealed in-ears physically tiring — the Apex vent addresses exactly that, and long-session comfort is the majority report
  • Buyers eyeing the Nio or the U12t who would rather keep the difference: reviewers repeatedly said the cheap one was the one they used
  • Anyone plugging into whatever is to hand — it is easy to drive and the flat impedance keeps the tuning honest across sources

Think twice if

  • You want vocals front and centre — the ear gain peaks early, near 2 kHz, and sopranos sit back for it
  • You are treble-sensitive: this is the axis reviewers split hardest on, and the top end can read glassy at volume
  • You need real isolation on a commute or a plane — a vented in-ear cannot match a sealed one, whichever module you fit
  • You are chasing maximum resolution at the price: the measurement-led reviews put it behind the sharper technical sets, its own U12t included
  • You have small or unusually shaped ears — the shell is a universalised custom mould with a long, narrow nozzle, so try before you buy
  • You want a balanced cable or premium accessories in the box: it ships 3.5 mm single-ended only, and spare modules are expensive

The reviewer consensus

Agreement &
disagreement.

5 / 12topics divide opinion
basstonalitydynamicsisolationimagingdetailvaluemidstreblesoundstagecomfortbuild
4 Strong agreement3 Mostly agree5 Divided opinions

Common ground9

  • The dynamic-driver bass is the headline and is praised almost without exception — deep, weighty and textured, with no bleed into the mids
  • Macro-dynamics are a standout: notes land with real force, which is what separates it from 64 Audio's all-BA sets
  • It is deliberately coloured, not neutral — bass-boosted and mildly U-shaped, sitting brighter than warm

Points of contention5

  • Treble: 'airy and extended without ever turning harsh' vs 'uneven and thin — glassy cymbals, fatiguing at volume.' The owner who measured it reports his own circle splitting love/hate
  • Midrange: everyone agrees the upper mids sit back; opinion splits on whether that reads as relaxed and non-shouty or as vocals robbed of presence
  • Soundstage: 'open and unusually tall' vs 'a smaller room where depth and width could be better' — and the writers on each side were using different Apex modules

A closer listen.

12 topics · 5 divided
Left: criticalRight: positiveTighter lines: more agreement

Bass

PositiveStrong agreement
Reviewer sentiment11 supporting sources
CriticalNeutralPositive

Band: opinion spread · Dot: weighted average

The single strongest point of agreement in the whole corpus, and the reason most reviewers recommend it: deep, weighty, textured, and kept clear of the mids. The only recurring reservation is character rather than quality — a minority hear the attack as soft and meaty rather than taut, which is also part of why the rest like it.

“Bass extension is excellent here and note also how it hedges the line between being a warmer tuning and a more sub-bass focused one.”

TechPowerUp (VSG)

“for a sense of authenticity to bass notes, the U4s certainly delivers with a meaty thickness and plenty of decay”

Headphones.com (Theo Lee / Precogvision)

“The texture is excellent, offering bass that doesn’t just hit hard, it also provides a clear tight presentation.”

Bloom Audio

“it sports a more pillowy characteristic to bass attack”

Headphones.com (Theo Lee / Precogvision)

Measured

Public IEC-711 exports put the bass shelf at +9.4 dB at 30 Hz over the ~450 Hz minimum with the stock m15 module, and +9.5 dB with m20 — the two bassiest modules differ far more in isolation than in bass. m12 drops it to +6.8 dB and mX to +3.0 dB, close to flat. TechPowerUp's independent rig read the m15 shelf at "~9.5 dB of bass boost with the m15 Apex modules here relative to the lowest point at ~500 Hz".

Link to this topic

The hardware

Specifications.

Checked

Design

Type
In-ear monitor
Drivers
1 × dynamic + 3 × balanced armature (1 tia high, 1 high-mid, 1 mid)1DD + 3BA
Housing
anodized aluminum shell, meteorite-inspired faceplate

Electrical

Impedance
11 Ω
Sensitivity
107 dB/mW≈ 126.6 dB/V
Frequency response
10 Hz – 20 kHz

Connection

Cable
Detachable
Connector
0.78 mm 2-pin
Plugs supplied
3.5 mm

Manufacturer-published figures, not measurements · from 64audio.com, listenup.com.

Follow the evidence

Sources 13

1–6 of 13 sources

Read with context

Limitations & method.

As of

What to keep in mind 7

  1. This is a read of where the reviews land as of September 2026. Most of them were written in the weeks around the March 2023 launch, and the kilobuck field has moved a long way since.

  2. Almost every opinion here was formed at $1,099. 64 Audio and most dealers now list the U4s at $1,199, so the value case is a little tighter than the quotes make it sound.

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